A conventional mobile communication system of the LTE (Long Term Evolution) scheme specified by the 3GPP is configured to communicate between a mobile station UE and a radio base station eNB, by using a certain key.
The certain key includes, for example, a key KRRC—Ciph used for “Ciphering” in an RRC protocol, which is a C-plane protocol between the mobile station UE and the radio base station eNB (Access Stratum, AS), a key KRRC—IP used for “Integrity Protection” in the RRC protocol, and a key KUP—Ciph used for a U-plane protocol between the mobile station UE and the radio base station eNB (Access Stratum, AS) and the like. These certain keys are generated using a first key KeNB.
Using the same key as any of the certain keys and the first key KeNB for a long time is not preferable, because it makes the system's security vulnerable. For this reason, a procedure for updating such a certain key or a first key KeNB during handover is devised by the 3GPP.
Here, operations of a handover target radio base station (Target eNB) acquiring a first key KeNB** used for generating a certain key in the handover procedure of the mobile station UE are described referring to FIG. 12.
As shown in FIG. 12, first, a handover source radio base station (Source eNB) generates an intermediate key KeNB* based on a stored first key KeNB, a parameter “Next Hop”, a parameter “Handover Type” representing the parameter type and a parameter “Target PCI” representing the identification information of a handover target cell.
Secondly, the handover source radio base station (Source eNB) transmits the generated intermediate key KeNB* to the handover target radio base station (Target eNB).
Thirdly, the handover target radio base station (Target eNB) generates, based on the the received intermediate key KeNB* and “C-RNTI (Cell Radio Network Temporay ID)” allocated by the handover target cell, a first key KeNB** used for generating a certain key in the handover target radio station (Target eNB).